Animal studies show that body position influences the development of ventilator-induced lung injury (VILI). We investigated whether the condition of patients may also contribute to VILI.In rabbits with normal lungs we evaluated the extent to which VILI changed dependent on body position, paralysis and ventilator settings. From 30 anesthetized and tracheostomized animals, 6 groups were randomly assigned according to ventilator settings (20 or 30 cm H_2O of peak inspiratory pressure, and 0 or 5 cmH_2O of PEEP), body position (prone or supine), and paralysis (with or without muscle relaxant). The animals were all ventilated with an infant ventilator (V.I.P.Bird) at a rate of 30/min, T_I 0.6 sec, and inspiratory flow 10 L/min in IMV mode. After mechanical ventilation began, respiratory gases were measured every hour. All animals were ventilated for 5 hours or until the PaO_2 value went below 100 mmHg at F_IO_2 0.5 and 60 mmHg at F_IO_2 0.21. At the end of the protocol, the lungs were removed.Results : In paralyzed animals in a supine position ventilated at PIP/PEEP 30/0 cmH_2O,PaO_2 decreased progressively as mechanical ventilation was applied regardless of F_IO_2. The lungs were reddish, especially in the dependent regions. In all the other groups of animals PaO_2 remained above 200 mmHg, and the lungs appleared normal.Conclusions : In the study, the development of VILI was influenced not only by the ventilator settings, but also by body position and the existence of spontaneous breathing. It is prudent for clinicians to keep in mind that these factors may greatly contribute to VILI.